Field of the Invention
The present invention relates generally to gas turbine engine combustor fuel injectors and, more particularly, to fuel injectors with multiple injection orifices and fuel purging.
Fuel injectors, such as in gas turbine engines, direct pressurized fuel from a manifold to one or more combustion chambers. Fuel injectors also prepare the fuel for mixing with air prior to combustion. Each injector typically has an inlet fitting connected to the manifold, a tubular extension or stem connected at one end to the fitting, and one or more spray nozzles connected to the other end of the stem for directing the fuel into the combustion chamber. A fuel conduit or passage (e.g., a tube, pipe, or cylindrical passage) extends through the stem to supply the fuel from the inlet fitting to the nozzle. Appropriate valves and/or flow dividers can be provided to direct and control the flow of fuel through the nozzle. The fuel injectors are often placed in an evenly-spaced annular arrangement to dispense (spray) fuel in a uniform manner into the combustor chamber.
Control of local flame temperature over a wider range of engine airflow and fuel flow is needed to reduce emissions of oxides of nitrogen (NOx), unburned hydrocarbons (UHC), and carbon monoxide (CO) generated in the aircraft gas turbine combustion process. Local flame temperature is driven by local fuel air ratio (FAR) in combustor zones of the combustor. To reduce NOx, which is generated at high flame temperature (high local FAR), a preferred approach has been to design combustion zones for low local FAR at max power. Conversely, at part power conditions, with lower T3 and P3 and associated reduced vaporization/reaction rates, a relatively higher flame temperature and thus higher local FAR is required to reduce CO and UHC, but the engine cycle dictates a reduced overall combustor FAR relative to max power. These seemingly conflicting requirements have resulted in the design of fuel injectors incorporating fuel staging which allows varying local FAR by changing the number of fuel injection points and/or spray penetration/mixing. Changing the number of fuel injection points is accomplished by shutting down some fuel circuits at part power. Fuel staging delivers engine fuel flow to fewer injection points at low power to raise local FAR sufficiently above acceptable levels for CO, UHC, and burnout. Fuel staging delivers engine fuel flow to more injection points at high power to maintain local FAR below levels associated with high NOx generation rates.
One example of fuel staging injector is disclosed in U.S. Pat. No. 6,321,541 and United States Patent Application No. 20020129606. This injector includes concentric radially outer main and radially inner pilot nozzles. The main nozzle is also referred to as a cyclone nozzle. The main nozzle has radially oriented injection holes that are staged and a pilot injection circuit which is always flowing fuel during engine operation. The fuel injector and a fuel conduit in the form of a single elongated laminated feed strip extend through the stem to the nozzle assemblies to supply fuel to the nozzle(s) in the nozzle assemblies. The laminate feed strip and nozzle are formed from a plurality of plates. Each plate includes an elongated, feed strip portion and a unitary head (nozzle) portion, substantially perpendicular to the feed strip portion.
Fuel passages and openings in the plates are formed by selectively etching the surfaces of the plates. The plates are then arranged in surface-to-surface contact with each other and fixed together such as by brazing or diffusion bonding, to form an integral structure. Selectively etching the plates allows multiple fuel circuits, single or multiple nozzle assemblies and cooling circuits to be easily provided in the injector. The etching process also allows multiple fuel paths and cooling circuits to be created in a relatively small cross-section, thereby, reducing the size of the injector.
Because of limited fuel pressure availability and a wide range of required fuel flow, many fuel injectors include pilot and main nozzles, with only the pilot nozzles being used during start-up, and both nozzles being used during higher power operation. The flow to the main nozzles is reduced or stopped during start-up and lower power operation. Such injectors can be more efficient and cleaner-burning than single nozzle fuel injectors, as the fuel flow can be more accurately controlled and the fuel spray more accurately directed for the particular combustor requirement. The pilot and main nozzles can be contained within the same nozzle stem assembly or can be supported in separate nozzle assemblies. These dual nozzle fuel injectors can also be constructed to allow further control of the fuel for dual combustors, providing even greater fuel efficiency and reduction of harmful emissions.
High temperatures within the combustion chamber during operation and after shut-down require the use of purging of the main nozzle fuel circuits to prevent the fuel from breaking down into solid deposits (i.e., “coking”) which occurs when the wetted walls in a fuel passage exceed a maximum temperature (approximately 400° F. or 200° C. for typical jet fuel). The coke in the fuel nozzle can build up and restrict fuel flow through the fuel nozzle rendering the nozzle inefficient or unusable.
To prevent failure due to coking the staged circuits should be purged of stagnant fuel and wetted walls either kept cool enough to prevent purge deposits (<550 F estimated non-flowing), or heated enough to burn away deposits (>800 F estimated), the latter being difficult to control without damaging the injector. Air available to purge the staged circuits is at T3, which varies so that it is impossible to satisfy either an always-cold or always-hot design strategy over the range of engine operation. A combination cold/hot strategy (i.e., use of a cleaning cycle) cannot be executed reliably due to the variety of end user cycles and the variability in deposition/cleaning rates expected.
Passive purging of fuel circuits has been used as disclosed in U.S. Pat. Nos. 5,277,023, 5,329,760, and 5,417,054. Reverse purge with pyrolytic cleaning of the injector circuits has been incorporated on the General Electric LM6000 and LM2500 DLE Dual Fuel engines, which must transition from liquid fuel to gaseous fuel at high power without shutting down. Stagnant fuel in the liquid circuits is forced backwards by hot compressor discharge air through all injectors into a fuel receptacle by opening drain valves on the manifold. This method is not suitable for aircraft applications due to safety, weight, cost, and maintenance burden. Forward purge of staged fuel circuits has been used on land based engines, but requires a high pressure source of cool air and valves that must isolate fuel from the purge air source, not suitable for aircraft applications.
Fuel circuits in the injector that remain flowing should be kept even cooler (<350 F estimated) than the staged circuit that is purging, as deposition rates are higher for a flowing fuel circuit. Thus, the purged circuit should either be thermally isolated from the flowing circuits, forcing the use of a cleaning cycle, or intimately cooled by the flowing circuits satisfying both purged and flowing wall temperature limits.
It is highly desirable to have a fuel injector and nozzle suitable for fuel staging using multiple circuit injectors with multiple point nozzles that require some circuits to flow fuel while other circuits in the same injector are purged. It is also desirable to have a suitable fuel injector and nozzle that allows the use of a valve in the injector to prevent shutdown drainage of supply tubes and to provide pressurization for good flow distribution at low fuel flows. It is very difficult to purge internal fuel circuits, requiring high flow rate of purge air that is relatively hot at some engine conditions. Thus it is highly desirable to cool the purge air to acceptable temperatures prior to entering the circuit being purged to prevent overheating the fuel conduit locally at the ingestion sites.